1. Field of the Invention
The present invention relates to an image processor, image processing method and image processing program, and more particularly to an image processor for eliminating the blurring of output images and acquiring output images faithful to input.
2. Description of the Related Art
Image processors, such as a printer, have been converting input image data, which have multi-value grayscale values for each pixel, into output image data having lower grayscale values (e.g. binary) by half tone processing, and printing the output image data onto print paper.
As a half tone processing, processing by a dot concentrated dither method (multi-value dither method) is known. In the multi-value dither method, thresholds are distributed so that dots grow from the center in a predetermined size of matrix, and processing is performed by comparing the thresholds with an input grayscale value of each pixel.
However if a threshold matrix is applied to the input image, in some cases the input grayscale value of the pixel is lost in an area where the threshold is high, and the input value is converted into an unnecessarily large output value in an area where the threshold is low. For example, if a fine line exists in the threshold matrix, the fine line in the output image may be disconnected. Therefore processing by the multi-value dither method has a problem in the image quality of the output image.
Therefore there is a prior art where a plurality of threshold matrices having different dot growth characteristics are provided, a matrix with which a desired dot size can be acquired is judged for each half tone generation area, and the threshold matrix to be used is switched (e.g. Japanese Patent Application Laid-Open No. H4-195137).
With this method however, a memory for recording a plurality of threshold matrices is required. Also the calculation volume is high since dither processing is performed for a plurality of times for all the pixels of the input image when the matrix is switched. Also in the case of processing by the multi-value dither method, including this method, the dot positions are fixed with a predetermined spacing, so a dot cannot be generated at an optimum position to reproduce the grayscale value distribution which was input, and low resolution images, which are not faithful to the input, may be output.
To solve these problems, the present applicant proposed a method of determining a center of gravity position from a grayscale value of each pixel in a cell which consists of a plurality of pixels, and generating a dot corresponding to the sum of the grayscale values of each pixel at the center of gravity position (e.g. Japanese Patent Application No. 2004-137326 (not published), hereafter called “AAM (Advanced AM Screen) Method”).
With the AAM method, however, dots are grown from the center of gravity position in a cell, so even if a fine line exists in the cell, for example, it is reproduced as a circular dot, and the output image becomes a blur.
With the foregoing in view, it is an object of the present invention to provide an image processor, image processing method and image processing program which can acquire an output image faithful to the input image, without blurring of the output image.